External Insulation Depth?

[mike2016]

I'm looking at getting external insulation fitting to my walls. It's a retrofit, 22 year old house, cavity block built construction. I was looking at 100mm but someone suggested 200mm. I've done some calculations as follows:

Current: U: 0.586

100mm EPS: 0.219

150mm EPS: 0.167

200mm EPS: 0.135

Now I could adjust the depth in places, deeper for north facing etc but it's a lot to add to the window reveals and then any fixings get a bit trickier. Any pros and cons / things I should be aware of? Thanks.

 

[A_L]

@mike2016 ,  in central England 100mm EPS would save about 16.1kWh/m² per year and the 200mm about 19.7kWh/m² per year (old SAP model)

[mike2016]

it's diminishing returns vs diminishing light through my windows!! I'm still thinking 100mm EPS gets me the best value / return and to not make it a pain to secure things to the wall / get service panel extensions, flue extensions, move the gas pipe too much etc. 

[Conor]

Are your cavities insulated? If so I'd expect a u value closer to 0.3?

 

I'd aim for an overall u value of 0.15 (as per passive House standard). any lower than this and you're getting deminishing returns.

Edited January 20, 2020 by Conor

[mike2016]

I've no cavities! Thought I had but I've solid block/brick lower front and cavity block everywhere else - you know those blocks with the two holes through the center of each side? So no cavity to pump. The only insulation is behind the dry lining internally, just some fibre glass and that's it. Thanks for the target - might be worth going for 150mm over 100mm then.....

[gravelld]

You only want to do this once. It will have to be done to the property eventually anyway to a decent standard. So either you do it, once, or someone else has to do it again. Good news for the installation companies I suppose.

 

IMO by far the biggest decisions revolve around enabling works. Foundations? Eaves and verges? Meter boxes?

 

Consider if this is an opportunity to establish an AT layer. I heard of some people painting the entire outside wall in blowerproof. Two coats (ideally three).

 

Having deeper reveals might be a good thing - will help with overheating.

[mike2016]

I'm going to attend a training class and see how I get on. I can do a small section of my house at the back, a protruding kitchen extension and if that flies over the handlebars I'll go bigger. I want to attend to the windows as I go and do the house in stages before I get to the front. At least that's the plan currently! Thanks for info.